Plate mounting tapes with foam film carriers are long-established and commercially commonplace. However, the production of mounting tapes with little fluctuation in thickness continues to be a fundamental problem, more particularly when the thickness tolerances are below ±10%. In many of the sectors in which double-sided foam adhesive tapes are employed, the printing industry being a particular example, low levels of deviation in thickness from the target figure, and low overall thicknesses, are a desideratum or a necessity. In the case of the flexographic printing process, for example, it is advantageous for the mounting of the polymer printing plate to the impression cylinder by means of a double-sided adhesive tape if that adhesive tape has a suitable resilience and low levels of fluctuation in thickness. If the resilience remains constant even under repeated compression of the foam film, then the foam film has an advantageous resilience and is preferred for use in this application. This is because it is possible in this way to achieve a considerable increase in print quality. A great problem in the production of plate mounting tapes to date has been the scant availability of foam webs possessing appropriate resilience.
When the printing operation is at an end, it must be possible to detach the adhesive tape, as far as possible without residue, both from the compression cylinder and from the plate, and this requires sufficient tensile strength. Used to date for this field of application have been adhesive tapes based on enhanced polyurethane foams (from Mayser, Lindenberg) or polyethylene copolymer foams or polyethylene-vinyl acetate copolymer foams (from Alveo, Lucerne). The PE-based foam films used possess the advantage over their aforementioned PU-based counterparts that they are obtainable with substantially lower compressive stress values. In respect of the deterioration in resilience as a result of fatigue of the material after numerous compression/recovery cycles as well, the closed-cell PE films are superior to the open-cell PU films.
A further requirement imposed on foam films used in plate mounting tapes is the temperature stability. At web speeds in flexographic printing of up to 600 m/min, the mounting tape may undergo warming to around 40 to 60° C. At 60° C., not only conventional PE foam films but also PU foam films display deterioration in resilience.
EP 0 491 253 A1 discloses a plate mounting tape with a foam film carrier material. The foam film is composed preferably of polyolefin, more particularly of polyethylene or of a polyethylene copolymer, with vinyl acetate, for example. The foam film may be oriented.
A foam web with a high fraction of polyethylene obtained from metallocene catalysis, and also an adhesive tape produced from a foam web of this kind, are described by EP 1 645 589 A1. The crosslinked foam web is produced by coextruding a thermally decomposing blowing agent and a polyolefin-based polymer in web form, crosslinking the coextruded product, and, finally, foaming it. The polymer contains at least 40% (w/w) of a polyethylene-based polymer prepared by metallocene catalysis by means of a tetravalent transition metal catalyst. A foam of EP 1 645 589 A1, despite crosslinking, and particularly at elevated temperatures, exhibits weaknesses in its resilience. A consistently high resilience, however, is a mandatory requirement in the context, for example, of use in an adhesive tape in the printing industry.
An ethylene multi-block copolymer is described by WO 2005/090427 A1. This multi-block copolymer is obtained from the polymerization of two olefins in the presence of two polymerization catalysts differing in their selectivity in respect of the two olefins, and also in the presence of a chain transfer reagent (“chain shuttling agent”). The resultant multi-block copolymer exhibits elastomeric behaviour at substantially higher temperatures than homogeneous copolymers comprising identical monomers.
WO 2005/090427 A1 also describes the production of a foam by the in-mould process from the ethylene multi-block copolymer. The product of the foaming operation is unsuitable for use as a foam film in a plate mounting tape, owing to the process and to the resultant properties. The required thickness, the low thickness tolerance, and the mechanical properties associated with the cell morphology and cell size cannot be achieved in this process. For these reasons the use of non-crosslinked polyolefin-based foam films or of foam films which have been chemically crosslinked and foamed simultaneously is not an option for plate mounting tapes.
It is an object of the invention to provide an adhesive tape which exhibits consistently high resilience at elevated temperatures and which therefore may be used more particularly as a plate mounting tape.
This object is achieved by means of an adhesive tape as described hereinbelow. The invention further relates to proposed uses of the adhesive tape of the invention.